Phase-locked loop circuits are a well-established tool in the communications and control fields. Phase-locked loops can be designed for a wide variety of desired characteristics including specifically, natural frequency, i.e., the characteristic response frequency of the loop when phase-locked, capture bandwidth, i.e., the range of bandwidth over which the loop will lock onto an incoming signal, and many others. Unfortunately, several of these parameters are interrelated so that, typically, in the design of a phase-locked loop, trade-offs must be made. For example, jitter can be reduced and the loop stabilized, but only at the expense of decreasing the loop bandwidth. This limits the range of frequencies over which the loop can acquire a signal, that is, lock onto and track a signal. Thus, it is not always possible to design a phase-locked loop so that its bandwidth is wide enough to capture all signals of interest.
The art, however, has provided two different solutions to the problem of achieving phase lock of signals outside the capture range of the loop. Of the two available solutions, one moves the input signal frequency to within the capture range of the loop, and the other moves the capture range of the loop to the signal frequency. The latter function can be accomplished in at least two different ways, ie., by using a frequency detector to control the loop to achieve phase-lock, or by using a frequency sweep circuit to vary the VCO frequency toward the signal frequency. Either approach will cause the capture range to move toward the signal frequency to enable phase-lock. However, both techniques require considerable additional circuitry and have special control requirements for enabling and disabling them. For example, either the frequency detector or frequency sweep circuit must be disabled when phase-lock is achieved and re-enabled when phase-lock is lost, and very importantly, the circuits must be disabled in such a way as to maintain phase-lock.
In more detail, FIG. 1 shows the frequency detector approach referred to above. A conventional second-order phase-locked loop comprises a phase detector 10, an active loop filter comprising an operational amplifier 15 and filter network 16 and a VCO 17. The output of the VCO 17 forms one input to the phase detector 10, the other input is provided by the input signal, i.e., the signal to which it is desired to lock. The output of the active loop filter is coupled to the VCO by an adder 18. A second input of the adder 18 is provided by the frequency detector 19, which has two inputs, the first being the signal input, and the second being the output of the VCO 17. Frequency detector 19 has several stringent requirements. It typically requires a continuous, unmodulated, single-tone signal to operate, and must maintain proper phase alignment with the quadrature detector when in phase-lock.
The other approach to providing a relatively wide capture range for a loop with inherently narrow bandwidth is shown in FIG. 2 in which similar elements have similar reference characters. In addition to the conventional phase detector, active loop filter, and VCO, a control path comprising a phase shift circuit 26, a phase detector 20, filter 21, comparator 22, driver 23, switch 24 and decay circuit 27 are provided to couple the sweep generator to the adder. The control elements detect lack of lock to close the switch 24 and thus the VCO frequency is swept under control of the sweep generator 25. When lock is detected, the switch 24 must be opened to prevent overdriving the VCO and a decay circuit allows the active filter to slowly replace the sweep voltage acquisition level.
Either the prior approach shown in FIG. 1 or 2 requires the addition of several active elements, as well as careful design to insure that the additional elements are enabled and disabled at the proper time and in such a fashion as to insure that the overall phase-locked loop function is not interfered with.
It is an object of the present invention to provide a simplified self-sweeping phase-locked loop which consists essentially of passive components in addition to the conventional phase-locked loop. It is a further object of the present invention to provide a self-sweeping phase-locked loop which automatically enters and leaves the sweeping mode of operation and maintains loop lock when leaving the sweeping mode of operation. It is still another object of the present invention to provide a self-sweeping phase-locked loop which is relatively simple, inexpensive and can be designed to exhibit a wide variety of characteristics.